As-worked bainitic ferrous alloy and method

ABSTRACT

THIS INVENTION RELATES TO AN AS-WORKED BIANITIC FERROUS ALLOY AND TO A NOVEL METHOD OF PROCESSING SAME TO OBTAIN OPTIMUM STRENGTH AND TOUGHNESS. MORE PARTICULARLY, THIS INVENTION IS DIRECTED TO THE HOT WORKING CYCLE OF A FERROUS ALLOY CHARACTERIZED BY AN I-T DIAGRAM OR S CURVE HAVING A DOUBLE NOSE OR A PEARLITE TRANSFORMATION KNEE OF THE BEGINNING CURVE ABOVE A BROAD BIANITIC BAY REGION. SUCH AN ALLOY IS HEATED TO AN AUSTENITIZING TEMPERATURE OF ABOUT 1500* TO 2200*F., AND SUBJECTED TO A PLURALITY OF WORKING OPERATIONS AT SUCCESSIVELY LOWER TEMPERATURES, WHERE THE FINAL WORKING OPERATION IS CONDUCTED AFTER THE BEGINNING OF THE AUSTENITE TRANSFORMATION TO BAINITE AND BEFORE THE COMPLETE TRANSFORMATION THEREOF.

TEMPERATURE April 23, 1974 B. L. BRAMFlTT ETAL 3,806,378

ASWORKED BAINITIC FERROUS ALLOY AND METHOD Filed Dec 20, 1972 Stongard Hot Rolled INVENTION TIME 3,806,378 AS-WORKED BAINITIC FERROUS ALLOY AND METHOD Bruce L. Bramfitt, and Arnold R. Marder, Bethlehem, Pa., assignors to Bethlehem Steel Corporation Filed Dec. 20, 1972, Ser. No. 316,962 Int. Cl. C21d 1/02, 1/20, 9/46 US. Cl. 148-12 7 Claims ABSTRACT OF THE DISCLOSURE This invention relates to an as-worked bainitic ferrous alloy and to a novel method of processing same to obtain optimum strength and toughness. More particularly, this invention is directed to the hot working cycle of a ferrous alloy characterized by an I-T diagram or S curve having a double nose or a pearlite transformation knee of the beginning curve above a broad bainitic bay region. Such an alloy is heated to an austenitizing temperature of about 1500 to 2200 F., and subjected to a plurality of working operations at successively lower temperatures, where the final working operation is conducted after the beginning of the austenite transformation to bainite and before the complete transformation thereof.

BACKGROUND OF THE INVENTION The invention, to be described in detail hereinafter, is directed to an as-worked bainitic ferrous alloy, and to a process including the thermomechanical treatment thereof. In general terms, bainite has been metallurigical- 1y defined as one of the transformation products of austenite. Upon cooling the austenite, transformation to bainite occurs over the temperature range of about 1000 to 450 F. The microstructure differs from pearlite, a high temperature austenite transformation product, in that it is acicular in nature. The last generally recognized transformation product is martensite. Transformation to martensite must be directly from austenite, i.e. without prior transformation to ferrite, pearlite or bainite. Unlike the transformation to pearlite or bainite, it is not time dependent, but occurs almost instantly upon cooling.

Over the recent years, the times, temperatures and rates of transformation of various steels have been determined and plotted-the result being a series of diagrams known as isothermal transformation diagrams, 'ITT diagrams, or S-curves. One current publication offering LT and GT or continuous cooling diagrams for standard steels, as well as modified versions thereof, is the Atlas of Isothermal Transformation Diagrams by US. Steel, 1963. It will be appreciated that the temperature and range noted above is not limiting but merely illustrative of a typical steel. A quick review of an I-T diagram for a given steel will give a closer reading of said steel. Nevertheless, while they are close, such diagrams are not precise as to exact times and temperatures.

For example, the LT diagrams were developed by studying the transformation behavior of a steel at a series of temperatures below the A critical temperature, by quenching small samples to the desired temperature in a liquid bath, allowing them to transform isothermally and following the progress of the transformation metallographically. From this, the curves could be drawn. Very generally, this procedure included heating, quenching, holding, and cooling. It has since been determined that processing variations such as working or rolling affected the actual time and temperature of transformation, particularly the pearlite transformation. More specifically it was discovered that the inclusion of an austenite working step changed the transformation kinetics of the steel by United States Patent ice shifting the curve to a higher temperature. This phenomena has been confirmed by Y. E. Smith in an article in Metallurgical Transactions, v. 2, June 1971. Nevertheless, with the apparent limitations, the LT diagram remains as a significant factor in selecting the proper ferrous alloy and for predicting results when subjected to the process herein.

With this background, consideration may now be given to certain aspects of prior art practices. Thermomechanical treatment of steel 'has long been considered as a means of improving mechanical properties. Several practices have been developed with the most well known of these being ausforming. This is identified and shown in the figure, where the practice is schematically illustrated superimposed on a typical I-T curve of a bainitic ferrous alloy. Isoforming is another practice where the ferrous alloy is worked as it isothermally transforms to pearlite. This, along with standard and controlled hot rolling, is illustrated in the figure in the manner of ausforming."

Variations to certain of these practices have been developed as evidenced by the following US. Pats. Nos. 2,717,- 846, 3,215,565, 3,240,634, 3,303,061, 3,340,102, 3,444,- 008, 3,453,152 and 3,645,801. While some of said patents treat alloys susceptible to processing by the method herein, none of them individually or in combination teach the present concept of subjecting the alloy in an austenitized condition to a plurality of working operations, preferably at least five, where the final working is concluded after the initiation of the austenite transformation tr;F bainite and before the complete transformation there- 0 BRIEF DESCRIPTION OF THE DRAWING The figure represents a typical I-T curve of a bainitic alloy susceptible to treatment by the process herein, with a number of schematic representations of thermomechanical treatment cycles superimposed thereover, comparing the present process to that of four prior art processes.

SUMMARY OF THE INVENTION This invention is directed to an as-worked bainitic ferrous alloy and to a procedure involving the thermomechanical treatment of the said alloy. More particularly it relates to a process of heating said alloy to an austenitizing temperature between about 1500 to 2200 F. and subjecting the alloy to a plurality of working operations as it cools from said austenitizing temperature. While the final working should be conducted during the transformation of the austenite to bainite, the initial working should begin while the alloy is above 1500 F. By this procedure, an optimum combination of strength and toughness, in the as-worked condition, is produced. Metallographically, the processed alloy is characterized by fine grains and cells, with very fine uniformly dispersed carbides.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT In the preferred practice of this invention, a bainitic ferrous alloy characterized by an isothermal transformation diagram having the pearlite transformation knee of the beginning curve laterally displaced to the right of the lower temperature bainitic transformation knee, is heated to an austentitzing temperature between about 1500 to 2200 F., preferably at least about 1600 F., and subjected to at least two working steps, preferably at least five, to reduce the cross section thereof, with the final of said steps occurring within the bainite transformation region.

Without intending to unduly limit the invention, a number of ferrous base alloys having the foregoing characteristic are listed with nominal compositions in Table I, by W i h e 7 TABLE I Each of said bainitic alloys A-W are characterized by a double nose I-T Diagram where the upper or pearlitic nose is laterally displaced to the right of the lower nose or region of bainitic transformation. For all of these alloys the end or tip of the pearlitic nose is somewhere between l200 and 1350 F. In other words, by the proper chemistry selection, i.e. balancing isothermal transformation retarding additions, and hardenability additions, it is possible to select an alloy possessing suflicient transformation time delays to permit working and cooling from the austenitizing temperature past the pearlitic nose, while permitting the bainitic transformation.

Returning now to the chemistry of the exemplary alloys of Table I, it will be noted that four elements (C, Mn, Mo and Cr) are conspicuous by their presence in all but two alloys. All of said elements are present in the alloys for their efiects on hardenability, and the overall effect in retarding isothermal transformation. The hardenability of a ferrous alloy is governed in large part by its chemical composition, and each addition or element present in the alloy affects hardenability to some degree. Much of this is known so it will suflice to say that the degree differs between additions, and between varying sub-ranges within a broad range of elemental addition.

In the present invention, carbon, while inherently present in steel, sho'uld be present in an amount sufficient to yield precipitatedcarbides upon final cooling. Generally, it is desirable to include from .10 to .50%, by weight, in the alloy, but a broader range of .03 to .65 by weight, may be used with success. As stated previously, all elemental additions aifect hardenability. This may be extended to say that all elemental additions, to a large or small degree, are effective in retarding isothermal transformation. For instance, a number of elemental additions, such as Mo, Cr, V, Si, Ti and Cb (identified for convenience as Group I elements), are effective in pushing the pearlite nose to the right. One of the most effective of these additions is molybdenum. However, the latter additions should be distinguished from some other elemental additions, such as Mn, Ni, B, N and Co, which act to shift the entire S curve to the right; these ma be identified as Group II elements. Thus, while one or more Group I additions are desirable, it is preferable that the alloy contain at least .25 by weight, molybdenum. But, when a lesser amount is used, it is desirable to use at least .75%, preferably, 1.00%, by weight, chromium. These elements (Cr and Mo) and theothers of Group I have been obsenved and are classified as ferrite stabilizers, and as noted above, all of said Group I elements retard the pearlite transformation, i.e., move the pearlite nose further to the right.

Considering now the procedural steps of this invention, it begins by heating a bainitic ferrous alloy to an austenitizing temperature of about 1500 to 2200" F. where the first working, such as rolling, should start. Several reductions or passes, preferably at least four, on the order of about 10% minimum for each reduction, should occur between the starting temperature and the onset of the bainitic transformation or the B temperature which occurs in the range of 900 to 1100 F. This sequence causes grain refinement of the austenite by repeated deformation and recrystallization. Additions of carbide forming elements may aid in the refinement of austenite grain size by precipitation in the austenite. At the lower temperatures, the recrystallization of austenite is retarded and the deformation causes substructural and textural development in the prior austenite grains. The further and final aspect of this procedure involves at least one additional pass below the bainitic start temperature to provide additional dislocations which act as nucleation sites for carbide precipitation. The latter is necessary to form numerous, fine precipitates which increase strength and improve notch toughness. Finally, this last reduction, in combination with the prior treatments, provides an optimum structure, i.e., fine grains and cells with very fine, uniformly dispersed carbides, and an optimum crystallographic texture for improved properties.

More particularly, the microstructure of the as-worked alloys of this invention consists of a fine, elongated grain structure, with a lath-like substructure and fine carbides, on the order of 0.01 to 0.03 am. in size, at the lath boundaries and dispersed within the laths. This structure is also associated with a dominant crystallographic texture of (111) [110], this designation is based upon the well known Miller Crystallographic Index System. *On rolled plates this texture is measured on the transverse plane, or perpendicular to the rolling direction. The intensity thereof ranges from 2.5 to 3.5 times random as measured by the nine-plane, inverse pole figure technique. A value of 1.0 represents a completely random orientation, whereas 9.0 shows or the orientation of a single crystal.

In contrast to this, the same alloy hot-rolled and finished above about 1600 F. reveals a microstructure of large equiaxed grains (ASTM No. 3-5). While there is no lathlike substructure, large carbides, on the order of 1 to 5 ,um. in size, are found within the grains and at the grain boundaries. This texture will nearly approach random with the intensity of any particular orientation below 2.5 times random, by the system noted above.

The improved properties of the alloys of this invention can best be demonstrated with rolling data and specific properties of six bainitic alloys treated by the method herein, and by the conventional hot rolling process, each of which are schematically illustrated in the figure. For convenience, the chemistry, by weight percent, for the bainitic ferrous alloys are given in Table II, with the rolling data and property results in Table III.

TABLE III Properties Rolling temp. F.) Transition Y. temp. Start Finish (k.s.i.) (CV-15 F.)

To effect a proper comparison between the processed steel alloys of this invention and the conventionally rolled steel alloys, all reductions from 4 inches to inch followed substantially one of two schedules set forth in Table IV.

K r g?elapse time for alloys finished above 1,590 F. was approximately 3 minutes; the elapse time for alloys finished between 1,190 700 F. ranged between and 26 minutes.

While temperatures were not ascertained following each pass for each alloy, where measured they were found to be linear, following a relatively constant cooling rate from the designated start to finishing temperature. For the alloys of this invention, the cooling rate varied between about 35-50 F./min. The foregoing is merely representative of reduction conditions, as variations from this may be made, particularly with changes in alloy composition. However, under all reduction practices, it is essential that at least the final reduction takes place within the bainitic region and concluded before the complete transformation of the alloy to bainite.

Returning to the results of Table HI, it will be evident that in nearly all situations wherein the method of this invention was followed, higher strength and improved impact properties were realized. Thus, with the procedure detailed herein, it is now possible to gain optimum mechanical and impact properties in as-rolled steels without resorting to costly heat treatments.

The attainment of optimum properties is clearly demonstrated with an exemplary comparison of Alloys AB3 and AB2. The yield strength was increased by at least 64% while the transition temperature was lowered from F. to 200 F. Another significant demonstration is noted when Alloy AB2 is compared to Alloy ABl, the latter having been finish rolled after complete transformation to bainite. While the precise critical temperatures of the worked alloy are not known, such an alloy under isothermal transformation conditions would reveal critical temperatures for the B and B, of approximately 920 F. and 750 F., respectively. The critical temperatures for the worked alloy would be slightly higher. Nevertheless, it is quite clear that even 750 F., the isothermal B temperature, is well above the finish temperature of 620 F. for Alloy ABl. While the Y.S. thereof remained at about the same level as the Y.S. for Alloy AB2, it will be observed that a significant loss occurred in the transition temperature. That is, the transition temperature was raised from 200 F. to 50 F.

While it can be expected that improvement will vary with the specific alloy being treated, it is believed significant that large improvements in strength and/or impact properties were noted in all situations.

We claim:

1. A method of producing a high strength ferrous alloy having a 01-15 notch toughness transition temperature less than about 25" F. in the as-rolled condition, said alloy characterized by an isothermal transformation diagram having the pearlite transformation knee of the beginning curve laterally displaced to the right of the lower temperature bainitic transformation knee, and consisting essentially, by weight, of about .03 to .65% carbon, a minimum of about .25 molybdenum, an addition of at least one element selected from the group consisting of boron, manganese, nickel, and chromium, balance essentially iron, said method comprising the steps of heating said alloy to an austenitizing temperature between about 1500" to 2200 F., and subjecting the alloy to a plurality of working steps which are conducted at progressively lower temperatures, and the final working step being conducted after the initiation of the austenite transformation to bainite and before the complete transformation thereof.

2. The method according to claim 1 wherein the first working step is initiated at a temperature between about 1600 to 2200 F.

3. The method according to claim 2 wherein there are at least four working steps prior to the onset of the bainitic transformation.

4. The method according to claim 1 wherein the carbon is present in an amount between about .10 to .50%

5. The method according to claim 1 wherein a suflicient quantity of a ferrite stabilizer is present in the alloy to retard the pearilite transformation.

6. The method according to claim 1 wherein each of said working steps reduces the cross section of the alloy by at least 10%.

7. The method according to claim 2 wherein there are up to ten working steps effecting a reduction of the cross section of the alloy.

References Cited UNITED STATES PATENTS 2,108,588 2/ 1938 Lawrence 148-12 3,423,252 1/ 1969 Grange 148-12 WAYLAND W. STALLARD, Primary Examiner U.S. Cl. X.R. 148l2.4 

